Data communications systems, such as xDigital Subscriber Line (xDSL), are generally required to provide for some form of protection from overvoltage conditions in data communication equipment, such as an xDSL modem. Lightning is a major source of overvoltage conditions on residential wiring. The overvoltage condition can result from a lightning strike or it can be induced in the AC transmission lines by a nearby lightning strike. Overvoltage conditions can also result from power line crosses caused, for example, by a vehicle striking a utility pole. Overvoltage conditions can also result from the loss of the AC neutral. If that occurs, devices that are normally connected to 110 to 120 volts AC are subjected to 220 to 240 volts AC. The loss of the AC neutral can result from corrosion of the meter pan at the service entrance where the meter is located outside the building and is exposed to the elements.
Primary overvoltage protection for telephone lines is provided by surge arresters located in network interface devices mounted on the outside of the building. Grounding for these overvoltage protection devices is provided by means of an earth ground brought into the enclosure at the time of installation and attached to ground bus.
Many users attempt to protect their valuable electronic equipment with plug-in surge suppressors. Such devices do not protect equipment from the large amplitude pulses caused by lightning strikes, although they do offer some protection against low energy transients originating within the home. Moreover, plug-in surge suppressors are generally located far from the earth ground where the electric utilities are brought into the building. A surge suppressor is only as good as the ground to which it is connected. Furthermore, most plug-in secondary surge suppressors being sold today are not designed to withstand the loss of the AC neutral and may create a fire hazard when the AC neutral is lost.